"Bald Eagle" <cre### [at] netscapenet> wrote:

I have a functional, if not yet rigorous understanding about the ray-radius
intersection test(s).  I am hoping to understand it completely enough to make
one of my explanatory diagrams.

> immediately following that, I have a comparison of the length of the ray
> direction vector D with some variable "a", which due to poor source code
> documentation, I have no idea what "a" exactly is, or why if they are equal that
> r0 has to get divided by the sqrt of a.
>
>     #if ((a = Dx * D.x + D.z * D.z) > 0.0)
>             #local r0 = r0/sqrt(a);
>     #end

I believe I see what's going on here.
in c++, == is a comparison.
= is an assignment.

So the #if line is simultaneously assigning the evaluation of the expression to
a, and then comparing it's value to 0.0.

> I also have Entry->A (through D) which I need to figure out.

This has to do with the arrow operator, or "Class Member Access Operator".

In sor.h, we have

struct Sor_Spline_Entry_Struct final
{
    DBL A, B, C, D;
};

and

struct Sor_Spline_Struct final
{
    int References;
    SOR_SPLINE_ENTRY *Entry;
    BCYL *BCyl;                 /* bounding cylinder.                  */
};

So "Entry" is a pointer to SOR_SPLINE_ENTRY, which has class members A through
D.

"Entry->A" is a whole identifier, which snakes through the 2 structs to identify
and use A.

and

class Sor final : public ObjectBase
{
    public:
        int Number;
        SOR_SPLINE *Spline;      /* List of spline segments     */
.... etc.

So A through D have something to do with the spline segments.
I just don't "see" where they come from or what their exact meaning is.
Perhaps somewhere else farther upstream in the code is a place where the control
points get grouped in sets of 4, and so A through D would be sets of control
points.

- BW

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